This invention relates to a method for removing impurities from a semiconductor substrate, and more particularly to a method for removing residual impurities existing in a surface silicon layer made of part of a silicon-on-insulator (SOI) substrate formed by ion implantation and a method for removing oxygen impurities existing in a silicon substrate.
(A) For example, the technology of SIMOX (Separation by Implanted Oxygen) for fabricating an SOI structure by implanting oxygen ions and making up a semiconductor device in this structure is explained below while referring to FIGS. 5 and 6. As shown in FIG. 5A, on the (100) plane 2A of silicon single crystal 2, .sup.16 O.sup.+ beam 4 is implanted by 1.times.10.sup.18 ions/cm.sup.2 with an energy of 80 keV, and it is annealed at 1150.degree. C. in a nitrogen atmosphere to form a buried insulator 6. Part 2B is a portion left over in the upper layer of the single crystal 2. Afterwards, As shown in FIG. 5B, an epitaxial layer 28 of about 1 .mu.m is laminated, and a specified semiconductor device 16 is formed on this layer 28 as shown in FIG. 5C (Y. Omura et al., VLSI Symposium, Kobe, 1985, 24-25). Numeral 18 is a LOCOS partly oxidized from 2B and 28, 26 is a gate electrode, 20, 22 are source and drain electrodes, and 24 is a gate insulator. The SOI substrate formed in this method is composed of, when the surface epitaxial layer 28 is removed actually in FIG. 5B, Si top layer b, layer 10 containing much oxygen as impurities due to radiation damage, and SiO.sub.2 layer 6 of correct composition ratio, sequentially from the surface side, as shown in FIG. 6 (P.L.F. Hewnent; Mat. Res. Soc. Symp. Proc., vol. 33, 1984, 41).
(B) Incidentally, when .sup.16 O.sup.+ beam is implanted by 2.25.times.10.sup.18 ions/cm.sup.2 with an energy of 150 keV and the wafer is annealed at 1250.degree. C. or higher temperature in a nitrogen atmosphere, the impurity oxygen in the silicon single crystal layer of the surface side is almost completely eliminated, and a steep interface is obtained (according to Mao et al., Appl. Phys. Lett., 48 (12), 24 Mar. 1986, 794).
(C) An attempt to modify the silicon surface by using hydrogen has been effected by annealing for 10 minutes at 950.degree. C. or higher temperature in Hz atmosphere (Y. Matsushita et al., Extended Abstracts of the 18th 1986 International Conference on Solid State Devices and Materials, Tokyo, 1986 pp. 529-532). According to this example, after annealing the silicon single crystal at 850.degree. C. to 1150.degree. C. in H.sub.2 atmosphere, it is further annealed at 1100.degree. C. for 16 hours in O.sub.2 atmosphere to induce defects, and the defects on the silicon surface are evaluated by Wright etching. When annealed at 950.degree. C. or higher temperature, there is almost no defect formed on the surface, and the dielectric strength of capacitors fabricated on these substrates are also enhanced from 950.degree. C. border line. This reason is that the oxygen concentration on the surface is lowered by H.sub.2 annealing.
In the conventional method (A), due to the radiation damage occurring at the time of ion implantation, a wide damaged layer 10 is left over, as shown in FIG. 5C, between the surface side silicon single crystal layer 28 and the SiO.sub.2 layer 6, and this layer 10 degrades the steepness of the Si-SiO.sub.2 interface, which may become a route of leakage current when a semiconductor device is formed on the surface silicon single crystal layer 28. Besides, the oxygen impurities left over in the silicon top layer 28 become a donor in the subsequent manufacturing process to give influences, such as fluctuations of threshold voltage of MOS transistor (David J. Foster et al., IEEE Trans. Ele. Dev., vol. ED33 (3), 1986, 354).
In the conventional method (B), since an abnormally high temperature treatment is needed in the semiconductor process exceeding 1250.degree. C., contamination by heavy metals from the annealing furnace materials may pose a serious problem.
In the high temperature annealing method in H.sub.z atmosphere in the conventional method (C), since pure hydrogen gas is used at a high temperature exceeding 850.degree. C., the risk is very high, and an apparatus particularly considered in his respect, such as furnace for epitaxial growth is necessary. Besides, because of thermal diffusion, it is hard to control the concentration, depth and other factors accurately, and it is hence difficult to obtain a low silicon defect layer in a desired range.